Field of the Invention
The present invention relates to a process for the preparation of alkyl mercaptans by reacting an aliphatic alcohol with hydrogen sulfide in the presence of a heterogeneous catalyst at varying temperatures.
Discussion of the Background
Alkyl mercaptans, also referred to as alkane thiols, are organic compounds with at least one covalently bonded thiol group (—SH) as functional group. From a formalistic point of view alkyl mercaptans correspond to alkyl alcohols in which the oxygen atom is replaced with a sulfur atom. Alkyl mercaptans are useful starting or intermediate compounds in the synthesis of important organic compounds. For example, methyl mercaptan is a very important intermediate compound in the synthesis of methionine as well as of dimethyl sulfoxide or dimethyl sulfone. Nowadays low molecular weight alkyl mercaptans such as methyl mercaptan is predominantly produced by reacting an aliphatic alcohol with hydrogen sulfide in the presence of a heterogeneous catalyst, which is typically based on aluminum oxide, and this reaction is performed in the gas phase at temperatures of from 250° C. to 500° C. and pressures of from 1 bar to 25 bar. However, besides the desired alkyl mercaptan, the reaction mixture also contains non-reacted starting materials and more importantly also by-products, such as dialkyl sulfide and dialkyl ether, as well as gases, which are inert under the used reaction conditions, for example alkanes, carbon monoxide, carbon dioxide, water and nitrogen, and in the case of aliphatic alcohols other than methanol olefins are also formed as by-products. Therefore, the desired alkyl mercaptan must be separated from these compounds, which is typically done by fractional distillation. Here, the largest cost factor is in particular the energy input which is necessary for the cooling of the reaction mixture in order to condense the alkyl mercaptan.
An economically competitive process for the preparation of alkyl mercaptans thus requires to keep the energy input for the separation of the alkyl mercaptan from the non-reacted starting materials and the by-products as low as possible. In practice, it is sought to achieve this goal through the highest possible selectivity for the formation of alkyl mercaptan or the highest possible conversion rate for the aliphatic alcohol or a combination of both.
Usually, catalysts on the basis of basic aluminum oxide are used as heterogeneous catalysts in the preparation of alkyl mercaptans. The addition of an alkali metal tungstate, for example potassium or cesium tungstate, to aluminum oxide typically leads to an increase in activity and selectivity of the catalyst for the formation of the alkyl mercaptan. The added alkali metal tungstates are often also referred to as promoter or active promoter. Catalysts with such promoters are typically prepared through impregnation techniques. The amount of tungstate, based on the total weight of the catalyst, is usually up to about 25 percent by weight, as described for example in the U.S. Pat. No. 2,820,062.
The amount of the alkali metal tungstate in the catalyst can be increased to a value of 25 percent by weight, based on the total weight of the catalyst, for example through the processes described in the published patent application EP 0 832 878 A2. However, these processes are relatively complicated and time-consuming because they involve a plurality of process steps: Aluminum oxide is impregnated with a solution of the specific alkali metal tungstate multiple times and each impregnation step is followed by drying at elevated temperatures or calcination.
A further increase in selectivity is observed for catalysts with an amount of more than 25 percent by weight, based on the total weight of the catalyst, of an alkali metal tungstate. Alkali metal tungstate concentrations of more than 25 percent by weight in the final catalyst are only possible when an impregnation solution is applied to the aluminum oxide, which contains the alkali metal in question and the tungstate in stoichiometric ratio of less than 2:1, as described in the published patent application DE 103 38 887 A1. However, the observed increase in selectivity is always accompanied by a decrease in activity.
In principle, it is possible to increase the loading of the catalyst with an alkali metal tungstate to values of even more than 35 percent by weight, based on the total weight of the catalyst, by use of an impregnation solution with a non-stoichiometric ratio of the alkali metal, for example cesium, and the tungstate. However, neither a significant increase in the conversion rate of the aliphatic alcohol nor an increase in the alkyl mercaptan selectivity was observed for catalysts with such high loadings of a tungstate. Rather, it was observed that the conversion rate of the aliphatic alcohol and the selectivity for the formation of the alkyl mercaptan even decrease for catalysts which are loaded with more than 45 percent by weight of tungstate, based on the total weight of the catalyst.
At best, catalysts obtained by impregnation techniques give alkyl mercaptan selectivities of about 95%, however only with a relatively low conversion of about 80% for the aliphatic alcohol.
As far as the optimization of the catalytic system is concerned, any further improvements in the alkyl mercaptan selectivity are therefore only possible through a change of the catalytic system, for example replacing the catalysts obtained by impregnation techniques with catalysts obtained by mixing techniques. The latter can be obtained by mixing aluminum oxide particles with an oxidic tungsten compound, such as tungstic acid or an alkali tungstate, and at least one separate alkali metal compound, as described in WO 2013/092129 A1. Using this process it is possible to load catalysts with more than 45 percent by weight of a tungstate, based on the total weight of the catalyst. These catalysts give methyl mercaptan selectivities of up to 97% and methanol conversion rates of up to 98%. However, these positive effects are only observed in the initial phase of the methyl mercaptan production. Then the methanol conversion rate and the methyl mercaptan selectivity rapidly drop and at the same time the formation of dimethyl ether, which is a by-product in the methyl mercaptan production, steadily increases. It is believed that the initially high values for the methanol conversion and the methyl mercaptan selectivity are caused by catalytically active phases containing dicesium tetrathiotungstate (W4Cs8S16). The drop in methyl mercaptan selectivity is believed to result from the formation of crystalline tungsten disulfide (WS2), which is less selective for the formation of methyl mercaptan than W4Cs8S16. It is further believed that preparing catalysts through mixing techniques, as described in WO 2013/092129 A1, only leads to a mechanic mixing of the alkali metal and tungsten, which however favors a growth of tungsten disulfide crystals. By comparison, the preparation of catalysts by impregnation techniques favors a mixing of the alkali metal and tungsten on an ionic level.
Alternatively, an improvement in the selectivity for the formation of alkyl mercaptans can also be achieved by increasing the molar ratio of hydrogen sulfide to alcohol. For example, a molar ratio of hydrogen sulfide to methanol of from 1:1 to 10:1 is usually employed in the preparation of methyl mercaptan. However, a high molar ratio of hydrogen sulfide to alkyl mercaptan necessarily results in a large excess of hydrogen sulfide in the reaction mixture and thus, the necessity to recirculate large quantities of gases back into the process. Since the recirculation of large quantities of gases requires a high energy input, the intention is to keep the excess of hydrogen sulfide as low as possible and at the same time achieve good alkyl mercaptan selectivities. In practice, the molar ratio of hydrogen sulfide to aliphatic alcohol therefore differs only slightly from 1:1. Consequently, the possibilities are limited for an improvement of conversion rates and selectivities through a modification of the molar ratio of hydrogen sulfide to alcohol.